Photovoltaic devices directly convert absorbed illumination to electrical energy. They are silent in operation and non-polluting and they are becoming an increasingly more important source of electric power. Tandem photovoltaic devices comprise a plurality of individual cells disposed on a common substrate in an optical and electrical series relationship. Light passes in sequence through the stacked cells and the voltages produced thereby are additive.
Tandem devices are generally more efficient than single cell devices since the thickness of the individual cells can be optimized to provide the highest operating efficiency and the use of a stacked array increases the utilization of all of the incident light. In some instances the band gaps of the active layers of the cells of the tandem device are varied so as to cause the upper-most device to absorb the short wavelengths of light and the lower cell to absorb the unabsorbed longer wavelengths of light. In this manner a relatively large portion of the solar spectrum may be usefully addressed.
Efficiency is a most important factor in any photovoltaic device and efficiency may be categorized as "operational" and "manufacturing." Operational efficiency is determined by the percent of incident light energy which a photovoltaic device converts to electricity. Obviously, efficiency should be as high as possible in order to minimize the area of a photovoltaic device needed to generate a given amount of power. Manufacturing efficiency is measured by the cost incurred in manufacturing the devices. Clearly, the two parameters interact. A high efficiency, very high cost device may be of lesser commercial importance than a low cost device of more modest efficiency.
Operational efficiency depends on both the quality of the photovoltaic material as well as the device configuration. Material quality is a measure of the density of defect states in a semiconductor material; material quality equates with the density of states in the band gap of the material. Material quality is difficult to measure directly, but it is readily correlatable with the performance of a device incorporating the material. Most photovoltaic devices undergo some degree of photodegradation in use which decreases their efficiency. The amount of photodegradation will vary, and it has been found that the quality of the material comprising the device will determine both the initial device efficiency as well as the likelihood that the device will degrade in use. Material quality has been found to be dependent, in at least part, upon preparation conditions. In the instance of thin film alloy materials such as glow discharge prepared Group IVA materials, deposition parameters such as power levels, gas pressures and deposition rate have been correlated with material quality. Configuration of the photovoltaic device is also determinative of operational efficiency. As noted above, tandem arrays of cells are frequently employed to produce a high efficiency device and, as will be described in greater detail hereinbelow, it is necessary to control the electrical parameters of the various cells comprising the tandem device so as to optimize current and voltage produced therefrom. Control of these parameters is particularly important when cells having different band gaps are disposed in a single device.
Manufacturing efficiency is strongly tied to the utilization of raw materials and to the speed at which devices are fabricated. In general, it is desirable to have maximum throughput in a manufacturing facility, and in the manufacture of photovoltaic devices this translates into rapid deposition of the component semiconductor layer. It has been found that photovoltaic materials produced at relatively high deposition rates are generally of lower quality than those produced at slower deposition rates; hence, high rates of production often represent a trade-off in device efficiency. To some degree, slow deposition rates can be compensated for by utilizing a larger deposition facility, albeit at increased overhead costs. It will thus be appreciated that in the manufacture of photovoltaic devices a practical balance must often be struck between operational and manufacturing efficiencies, particularly in those instances where high volume, relatively low cost consumer oriented products are being produced. Therefore any method or device configuration which can increase either the operational or manufacturing efficiency of a photovoltaic device without adversely affecting the other parameter will be of significant commercial importance.
The present invention, as will be fully explained hereinbelow, provides a relatively high speed, efficient process for the manufacture of tandem photovoltaic devices having high operational efficiencies. The present invention also has great utility in the manufacture of high volume consumer oriented photovoltaic devices such as those used for power generation. These and other advantages of the invention will be readily apparent from the drawings, discussion and description which follow: